Ian Jolly

Global impacts of conversions from natural to agricultural ecosystems on water resources: Quantity versus quality

[1] Past land use changes have greatly impacted global water resources, with often opposing effects on water quantity and quality. Increases in rain-fed cropland (460%) and pastureland (560%) during the past 300 years from forest and grasslands decreased evapotranspiration and increased recharge (two orders of magnitude) and streamflow (one order of magnitude). However, increased water quantity degraded water quality by mobilization of salts, salinization caused by shallow water tables, and fertilizer leaching into underlying aquifers that discharge to streams. Since the 1950s, irrigated agriculture has expanded globally by 174%, accounting for � 90% of global freshwater consumption. Irrigation based on surface water reduced streamflow and raised water tables resulting in waterlogging in many areas (China, India, and United States). Marked increases in groundwater-fed irrigation in the last few decades in these areas has lowered water tables (� 1 m/yr) and reduced streamflow. Degradation of water quality in irrigated areas has resulted from processes similar to those in rain-fed agriculture: salt mobilization, salinization in waterlogged areas, and fertilizer leaching. Strategies for remediating water resource problems related to agriculture often have opposing effects on water quantity and quality. Long time lags (decades to centuries) between land use changes and system response (e.g., recharge, streamflow, and water quality), particularly in semiarid regions, mean that the full impact of land use changes has not been realized in many areas and remediation to reverse impacts will also take a long time. Future land use changes should consider potential impacts on water resources, particularly trade-offs between water, salt, and nutrient balances, to develop sustainable water resources to meet human and ecosystem needs.

Unsaturated zone tritium and chlorine 36 profiles from southern Australia: Their use as tracers of soil water movement

In this paper we present seven unsaturated zone profiles of 36Cl and three 3H profiles from southern Australia. All profiles show single peaks corresponding to high radionuclide fallout from nuclear testing in the 1950s and 1960s. The profiles are used to estimate rates of water movement leading to recharge of the groundwater. Among these profiles is the first profile on which high concentrations of 36Cl have been found below a 2-m depth. In this profile, 3H and 36Cl peaks coincide. In six of the seven profiles, total 36Cl fallout was found to be between 1.2 and 2.4 × 1012 atoms m−2, and is of similar magnitude to that found at similar latitudes in the northern hemisphere. Comparisons of soil water fluxes estimated with 3H, 36Cl, and chloride are briefly discussed.

Impact of flooding on the water use of semi-arid riparian eucalypts

The water use strategy of Eucalyptus largiflorens (F. Muell.) was investigated in response to flooding on the Chowilla Anabranch, a semi-arid floodplain of the Murray River, South Australia. Water use was measured using the heat pulse technique at six sites that varied in flood duration from 0 to 78 days. Soil chloride, plant water potential and surface root mass were also measured. Suppression of tree water use did not occur during flooding regardless of flood length and site health, suggesting that sufficient oxygen had been available to the trees. Increases in tree water use occurred at some sites after the flood because of increases in water availability due to leaching of salt from the soil profile. The soils with a higher clay content incurred little leaching of salts and therefore little change in tree water availability. In contrast, the sites with more sandy soils encountered greater leaching and greater increases in tree water availability. Despite differing soil type responses, all tree communities investigated showed a reduction in tree water stress in the period after flooding. These findings suggest that flooding in this environment improves the health of Eucalyptus largiflorens in the short-term. The implications of these findings are discussed with regard to the management of the Chowilla Anabranch.

Modelling vegetation health from the interaction of saline groundwater and flooding on the Chowilla floodplain, South Australia

The native riparian vegetation communities on the Chowilla floodplain in the lower River Murray in South Australia are suffering severe declines in health, particularly the Eucalyptus camaldulensis Dehnh. (red gum) and Eucalyptus largiflorens F.Muell. (black box) communities. The primary cause of the decline is salinisation of the floodplain soils caused by increased rates of groundwater discharge and hence increased movement of salt up into the plant root zone. The salinity is driven by a lack of flooding and rising saline groundwater tables. Rises in the naturally saline groundwater levels are due to the effects of river regulation from Lock 6 and high inflows from regional groundwater levels increased by Lake Victoria to the east. River regulation has also led to reduced frequency and duration of the floods that leach salt from the plant root zone and supply fresh water for transpiration. The frequency of medium-sized floods occurring on Chowilla has been reduced by a factor of three since locking and water extractions were commenced in the 1920s to provide reliable water for urban and agricultural use. The soil salinisation on the floodplain was modelled by using a spatial and temporal model of salt accumulation from groundwater depth, groundwater salinity, soil type and flooding frequency. The derived soil water availability index (WINDS) is used to infer vegetation health and was calibrated against current extent of vegetation health as assessed from fieldwork and satellite image analysis. The modelling work has shown that there is a severe risk to the floodplain vegetation from current flow regimes. This paper estimates that 65% (5658 ha) of the 8600 ha of floodplain trees are affected by soil salinisation matching a field survey of vegetation health in 2003 (Department of Environment and Heritage 2005a), compared with 40% in 1993 (Taylor et al. 1996). Model results show that the best management option for Chowilla is lowering the groundwater down to 2 m below current levels, which predicts an improvement in the health of the floodplain tree species from 35 to 42%.

The impact of flooding on modelling salt transport processes to streams

Abstract The development of many of the worlds arid and semi-arid regions has resulted in the salinisation of land and water resources. In these areas, soils and groundwaters are often naturally saline and any disturbance of the delicate hydrological balance results in mobilisation of the stored salt. The salt transport mechanisms are often highly complex, the understanding of which necessitates the use of computer modelling in combination with field studies. In this paper the transport of salt between groundwater and streams on the Chowilla floodplain in south-eastern Australia was modelled and compared with available field data. The large salinity contrast between the fresh stream and floodwater and the saline groundwater results in density-dependent flow behaviour, and hence necessitated the use of a variable density flow and solute transport model (SUTRA). The model was applied in cross-section over a 6.1-km-long transect across the floodplain. Time varying boundary conditions were employed at the locations of three streams on the transect to simulate the interaction between the rising and falling streams and the adjacent aquifer during and after floods. The model was used to assess the importance of overbank floods in the transport of salt to floodplain streams by carrying out simulations under various recharge scenarios. The simulations showed that the mixing of floodwater and groundwater within the bank storage adjacent to the streams could predict the observed short-term (

From drainage to recharge to discharge: Some timelags in subsurface hydrology

Abstract In arid areas, low rates of groundwater recharge and thick unsaturated zones mean that significant time lags can occur between changes in drainage below plant root zones and changes in aquifer recharge. The magnitude of the timelag can be approximated using simple analytical expressions, or more accurately determined using numerical models of soil water movement. It is related to the thickness of the unsaturated zone, the magnitude of the drainage rates, and the soil hydraulic properties. In southern Australia, in areas where water tables are 30 m below the land surface, timelags of 30–200 years occur between clearing of native vegetation for agriculture and an increase in groundwater recharge. It is predicted that similar timelags would occur between revegetation and a reduction in recharge. The timelag between a change in groundwater recharge and the change in groundwater discharge is related to the distance between the recharge and discharge areas, and the aquifer transmissivity and specific yield. For large groundwater basins with low recharge fluxes, this timelag can be many hundreds or thousands of years. In some cases, this allows groundwater extraction at rates well in excess of recharge rates to continue for a number of years before the impact of this policy will be seen. On the other hand, in southern Australia revegetation to reduce recharge and hence lower watertables and reduce saline groundwater flows to one of Australias most important rivers (the Murray River) are being considered. The timelag between the reduction in recharge and the consequent reduction in discharge limits the effectiveness of this management option in areas further than a few kilometre from the river.

A flood history weighted index of average root-zone salinity for assessing flood impacts on health of vegetation on a saline floodplain

Abstract Native Eucalyptus woodlands on the floodplains of the lower River Murray, Australia are dying because of increased soil salinity associated with shallower watertables and reduced flooding. There is need for a simple salinity index, related to vegetation health, which can be used within a geographic information system (GIS) to evaluate the potential impacts across the floodplain of management options which aim to decrease soil salinity. Management options include increasing the frequency and duration of floods by releasing additional environmental flows from upstream storages or lowering the watertable by groundwater pumping. This paper extends a simple root-zone salt and water balance model which represents the salinisation process as a moving salt front (MSF) [Jolly, I.D., Walker G.R., Thorburn P.J., 1993. J. Hydrol. 150, 589–614; Thorburn, P.J., Walker, G.R., Jolly, I.D., 1995. Plant and Soil 175, 1–11], to develop a flood history weighted net discharge salinity index (WINDS-Index) which relates to vegetation health. The MSF model, based on steady state groundwater discharge theory incorporating water uptake by vegetation, is evaluated against simulations made using a fully dynamic soil-vegetation-atmosphere model (WAVES). These simulations evaluated the impact of groundwater pumping and flooding options [Slavich, P.G., Walker, G.R., Jolly, I.D., Hatton, T.J., Dawes, W.R., 1999. Agric. Water Manage. 39, 241–261]. The dominant features of the WAVES simulations were adequately reproduced using the moving front model, provided the discharge rate was limited to a potential canopy transpiration rate. The WINDS-Index reflects the impact of flooding history on the long term average soil water salinity for soils with varying hydraulic properties, watertable depth and watertable salinity. The WINDS-Index is strongly dependent on the relative inundation time, defined as the ratio of the duration of inundation to the duration between floods, of successive flood events and has application as a management tool within a GIS. The watertable depth for long term control of root-zone salinity is defined using the concept of a critical salt balance criterion which incorporates the relative inundation time and soil hydraulic properties.

Groundwater modelling to quantify saline inflows to the River Murray and to optimise salt interception schemes near Waikerie, South Australia

Abstract The Murray-Darling Basin Salinity Management Strategy (BSMS) aims to control natural salt loads and the rise in land use change-induced salt loads in the River Murray and tributaries, and protect key natural resource values. Many cities and towns in South Australia source between 50% and 100% of their water supplies from the River Murray, and increasing salinity has major cost implications for infrastructure, as well as for potable water quality. The BSMS recognises that measures to counter salinity such as water use efficiency and revegetation will take decades to become effective, hence the need for large scale groundwater pumping schemes to intercept saline inflows. The BSMS and various State legislation and strategies require the use of models to quantify salt loads to the River and to attribute accountable actions to the appropriate responsible parties. Salinity Registers track the balance of the debits and credits, to manage future changes in salt loads. This paper demonstrates how groundwater modelling is an effective tool for both quantifying and optimising the design and operation of salt interception schemes, and also for differentiating the flow contribution and timing of saline inflows due to natural flows and changed land use impacts. Thus modelling can inform the process of quantifying contributions due to the “legacy of history” (ie. pre-1988 impacts) and due to “future impacts” (post-1988 accountable actions).